Impact sensitive security window system

ABSTRACT

A security window system having a transparent structure having high resistance to penetration is described. A transparent conductive layer is provided over most of the area of the window and the resistance of the layer is monitored for sensing penetration. Preferably the layer is subdivided into a number of conductive regions for substantially increasing the sensitivity of the system to minor interruptions in the layer. Temperature and stress effects can be minimized by connecting different conductive areas of the layers as arms of a resistance bridge. An alarm may be sounded when a small steady state change in resistance is sensed or when a rapid change in resistance is sensed.

United States Patent [191 Laidlaw, Jr. et al. v

[ July 23, 1974 IMPACT SENSITIVE SECURITY WINDOW SYSTEM [75] Inventors:H. Gordon Laidlaw, Jr., Thousand Oaks; Berton P. Levin, Santa Monica,both of Calif.

[73] Assignee: The Sierracin Corporation, Sylmar,

Calif.

[22] Filed: Nov. 16, 1972 21 Appl. No.: 307,090

[52] US. Cl 340/274, 52/616, 109/21, 161/404, 340/285 [51] Int. Cl. G08b13/04 [58] Field of Search 340/274, 285; 109/21, 10, l09/49.5; 52/171,616; 161/192, 404

[56] References Cited I UNITED STATES PATENTS 2,864,928 12/1958Danford... 52/171 .2,921,257 111960 Boicey 340/274 3,180,781 '4/1965'Ryan et al 161/192 7/1971 Ham et al. 340/285 6/1972 Littell, Jr.109/49.5

Primary Examiner-John W. Caldwell Assistant Examiner-Glen R. Swann, IllAttorney, Agent, or Firm-Christie, Parker & Hale [5 7 ABSTRACT Asecurity window system having a transparent structure having highresistance to penetration is described. A transparent conductive layeris provided-over most of the area of the window andthe resistance of thelayer is monitored for sensing penetration. Preferably the layer issubdivided into a number of conductive regions for substantiallyincreasing the sensitivity of the system to minor interruptions in thelayer. Temperature and stress efiects can be minimized by connectingdifferent conductive areas of the layers as arms of a resistance bridge.An alarm may be sounded when a small steady state change in resistanceis sensed or when a rapid change in resistance is sensed.

11 Claims, 13 Drawing Figures IMPACT SENSITIVE SECURITY WINDOW SYSTEMBACKGROUND poration, assignee of this application.

In many situations it is desirable to have a transparent window that isrelatively impenetratable. Such windows may be used in prisons,hospitals, museums, zoos, computer rooms, laboratories, or in storefronts where theft or vandalism may be a problem.They are useful anyplace where maximum naturallighting, visual access and physical securityare requisite. Jewelry countersand laboratory hoods are other suitablelocations. Bars can be added adjacent ordinary glass; however, this isin many cases undesirable for a variety of reasons. Thus, in a prison orhospital or similar institutions, bars may have a significantlyundesirable effect on persons within the institution. Bars detract fromthe pleasure of visitors to zoos or museums. In stores and the likewhere protection is desired against entry, the presence of bars ishighly undesirable because of the adverse effect of potential customers.Collapsible window grates are little better.

Windows that are highly resistant to penetration can be formed withthick layers of glass or preferably with laminated glass sandwicheswhich may include layers of tough, impact resistant plastics, such asthe polycarbonate plastics. Tempered glass is desirable in somesituations in case of breakage. Thus, for example, in some institutionspersons may deliberately break windows to obtain slivers of glass to useas weapons or to ingest in a suicidal act. Tempered glass is desirablefor suchsituations since it does not shatter like ordinary glass butbreaks into relatively small fragments substantially free of all sharpedges.

In addition to resistance to penetration it is often highly desirable toprovide sensing of efforts to penetrate so thatan alarm can be soundedlocally or at some remote station. Thus, for example, penetration of aprison window indicates either an escape attempt or an effort to conveycontraband. Sensors in the individual prison windows can be monitored ina central location for detection of such unlawful activities. Similarly,in stores or the" like, breakage of a window commonly precedes aburglary attempt. For this reason, burglar alarm systems commonlyinclude means for sensing breakage of thewindow.

A verycommon technique for sensing breakage of a window is to-adhere aconductive tape such as thin aluminum or lead foil directly to the glassaround the periphery of the window. Such strips are unsightly and arepreferably avoided, particularly in store windows and the like where anattractive appearance is highly desirable. Omission of the obvious alarmstrips may also be desirable in some institutional windows. The alarmstrips have another disadvantage in that they are essentially a-binarydevice that is either intact or broken. When such a tape is broken thealarm system is inoperative until someone gets to the window ,andbridges a break in the tape. There is no way of resetting such an alarmfrom a remote location.

It is therefore desirable to provide a security window system having analarm built into it which is sensitive to attempts to penetrate thewindow and which can be reset from a remote location. It is desirable tohave a signal from the window that is related to the degree ofpenetration, which in this context can be considered to be an analogchange as compared with the binary change that occurs upon completeinterruption of an electrical path. Preferably such a security window issubstantially free of apparent visual indications of the presence of thealarm. For most uses the window is preferably resistant to penetrationwith the impact resistance of polycarbonate and the resistance to sawingthat is characteristic of .glass.

BRIEF SUMMARY OF THE INVENTION of the window and is connected to meansfor giving an output signal in response to a transient change inresistance inexcess of some predetermined magnitude,

' Preferably the predetermined magnitude is adjustable for detecting astrain in the window that is. a predeter mined fraction of the strain atwindow penetration.

DRAWINGS:

These and other features and advantages of the present invention will beappreciated as the same becomes better understood by reference to thefollowing detailed description of presently preferred embodiments whenconsidered in connection with the accompanying drawings wherein:

FIG. 1 is a face view of a security window including an alarm sensor;

FIG. 2 is a fragmentary cross section of thewindow of FIG. 1; r

FIG. 3 is a schematic diagram of a sensing circuit for the window ofFIG. 1;

FIG. 4 is a face view of another embodiment of security window;

FIG. 5 is a block diagram of a sensing circuit for the window of FIG. 4;

FIG. 6 is a face view of another embodiment of security window;

FIG. 7 is a face view of still another embodiment of security window;

FIG. 8 is a partially cut-away section of the window of FIG. 7;

FIG. 9 is a fragmentary cross section of another embodiment of securitywindow;

FIG. 10 is a fragmentary cross section of another embodiment of securitywindow;

FIG. 11 is a schematic diagram of another penetration sensing system;

I FIG. 12 is a schematic diagram of another resistance sensingtechnique; and r FIG. 13 illustrates another embodiment of securitywindow and a schematic circuit connected thereto.

DESCRIPTION FIG. 1 is a face view of a security window and FIG. 2 is afragmentary cross section showing the laminated layers thereof. In faceview the security window appears much like an Ordinary transparentwindow except that it may appear somewhat tinted or have slightly lesslight transmission than an ordinary clear glass window. In addition,very narrow isolation lines 20, described in greater detailshereinafter, may be seen in the face of the window. Ordinarily theselines are very minute and not noticeable except onclose examination.Metallic bus bars 21 are imbedded' along opposite side edges of thesecurity window. A short-tab 22 from each of the bus bars typicallyextends beyond the edgeof the window formaking electrical contact. Thebus bars are the security window are illustrated in the fragmentary viewof FIG. 2. A sheet of tempered glass 23 forms one face of the window. Aswill be apparent thereinafter it is preferred that this face be the onefrom which penetration is 'most likely to occur. Typically the temperedglass layer is about one quarter inch thick. Atranspar ent resilientplastic interlayer 24 is securely bonded to the glass sheet 23. Thisinterlayer is the same as that typically employed in laminatedautomobile glass, for example. A layer 0.030 inch thick of polyvinylbutyral makes a suitable interlayer that is conveniently bonded to theother layers of thelaminated window by conventional heat and pressurelaminating techniques.

A carrier film 25 having a metal layer 26 on one face thereof is bondedto the plastic interlayer 24. It is relatively unimportant which face ofthe carrier film'has the metallic layer thereon. The carrier film is,for example, a film of polyethylene terephthalate about 0.005 inchthick. The metal layer 26 is an extremely thin layer of a metal such asnickel, gold, silver, aluminum, copper or the like which can be vacuummetallized onto the carrier film; Such'vacuum deposition of thin metalfilms is a conventional process widely used for preparing electricallyheatable windows. The metal coating is deposited in a sufficiently thinlayer that it is transparent and absorbs relatively minor amounts ofincident light so that the overall transmission characteristicsof thewindow are not substantially diminished. The metal layer is sufficientlycontinuous to have a substantial electrical conductivity.

By employing a thin carrier film the metal layer may be vacuum depositedon the carrier film by a continu ous process whereby large sheets ofcarrier film are coated and subsequently cut to a desired size.Relatively uniform resistance throughout the metal layer can be achieverby such a vacuum metallizing'treatment. The conductive layer 26 extendsover most of the area of the security window. If desired for inhibitingenvironmental access to the metal layer it may be deleted from theperipheral areas of the carrier film. About the only requirement is thatthe conductive layer extend near enough the edges of the security windowto make good electrical contact with the bus bars 21 (FIG. 1) adjacentthe edges of the sheet and extend over most of the area of the windowwhere penetration may be likely to occur. Ths bus bars are imbedded inthelaminate between the interlayer 24 and the carrier film 25 so as tobe in electrical contact with the metal film.

Another inlerlayer 27 is bonded to the opposite side of the carrier film25 from the first interlayer 24. These interlayers are substantiallyidentical. An impact resistant plastic ply 28 is bonded to the secondinterlayer 27. A variety of transparent impact resistant plastics aresuitable'for use in such a security window. Methyl methacrylate resinmay be employed, for example. It is preferred, however, to employ apolycarbonate resin for the plastic ply. This material is commerciallyavailable under-the trademark Lexanfrom General Electric and under thetrademark Merlon from Mobay Chemi-' cal Company.'The polycarbonate sheetis extremely impact resistant and has a high transparency. Thus, even ifthe tempered glass layer 23 is broken the polycarbonate layer 28normally resists impact penetration. Such a polycarbonate sheet, forexample, may be about one quarter inch thick. 7

Another polyvinyl butyral layer 29 is bonded to the other side of theimpact resistant ply 28. This interlayer is substantiallyidentical tothe first two. Finally a second sheet 30 of tempered glass is.bonded tothe third interlayer29 and forms the other face of the laminatedsecurity window. This second layerof tempered glass is also about onequarter inch. The glass and plastic layers have differing coefficientsof thermal expansion and when temperature cycling is expecteditisdesirable to provide stress relief around the periphery of the window.A suitable edge separator technique is provided in copending US. Pat.application Ser. No. 1 11,993 by Jan B. Olson, entitled InterlayerStress Reduction in Laminated Transparencies and assigned to SierracinCorporation, assignee of this application. The polycarbonate layer maybe subject to attack by plasticizers in the polyvinyl butyral layer andit is usually desirable to employ a polycarbonate sheet-with a barrierlayer on its faces. Such a coated polycarbonate material is availablefrom General Electric under their trade designation MR-4000. Any of avariety of conventional transparent melamine, phenoxy or urethane resinsform suitable barrier layers.

The security window illustrated in FIG. l'is highly resistant topenetration since the tempered glass has substantial impact resistance.Even if the glass layer is broken by scratching or sharp impact thepolycarbonate layer has much higher strength and ordinarily hassufficient impact resistance to prevent penetration. Tempered glassbreaks into a large number of relatively small particles and theseparticles remain bonded to the interlayer. The presence of such a massof glass fragments on the surface of the window does a great deal toinhibit sawing or other cutting of the polycarbonate plastic.

There are substantial advantages to having a security window formed witha glass face layer and a'polycarbonate plastic layer laminated togetherin combination with an alarm as herein described. It will be noted thatthe carrier film where the conductive layer'forming the analog sensor ofthe alarm circuit is located is separated from the glass andpolycarbonate layers by a relatively soft and flexible polyvinylbutyralinterlayer. If the frangible glass layer is broken, as by a sharplocalized blow or a deep scratch which may trigger fracture of temperedglass, the glass fragments are largely held in place by adhesion to theinterlayer. Thecracks from the glass seldom penetrate the resilientinterlayer and hence do not interrupt the thin metal film. Thus the mereface that the glass is broken does not necessarily trigger an alarm. Thesame is not-true of a system wherein the alarm sensor comprises a leadtape around the periphery of the window or an electrically conductivefilm applied directly to the glass. In such a system a crack propagatingto the edge of the window normally results in breaking of the lead tapeor conductive film and triggering of an alarm.

The resistance sensor extending over most of the area of the windowtherefore serves to detect penetration of the window. When a hole ismade in the window of a sufficient size to interrupt a portion of theconductive layer, the alarm will be triggered. Mere cracking of thewindow or surface damage will not ordinarily trigger the alarm.Detection of penetration is what is sought and this is provided by thecomposite laminated-window with a conductive layer embedded therein.

Referring again to FIG. .1, the conductive layer is in electricalcontact-with the bus bars 21 along opposite edges of the window. Aresistive connection is thereby provided between the two bus bars. Theisolation lines are actually extremely fine scribe lines made in theface of the carrier film on which the conductive layer is deposited.Since this conductive layer is extremely thin a scribe line that isnearly invisible to the naked eye is sufficient for interrupting theelectrical continuity of the film. A scribe line can be made with ashallow sharp groov that extends into the carrier film a tiny distance,but not even this is needed. The metal layer is so thin that almost" anyabrasion is enough to interrupt it without marring the carrier film.

If an effort is made to penetrate the security window the electricallyconductive layer must also be penetrated. Any interruption of theconductive layer having a component in a direction parallel to the busbars will cause an increase in the resistance of the conductive layer.As pointed out hereinafter the resistance between the two bus bars 21can be monitored and any significant change in resistance employed fortriggering an alarm. Any such sensing system has a predeterminedsensitivity. If the sensitivity threshold for triggering an alarm is toosmall, a significant number of false alarms may be sounded. On the otherhand if the threshold of sensitivity for triggering the alarm is toohigh, a rather large penetration of the window may occur before an alarmis sounded. It has been found that a sensitivity threshold in the areaof about one to two percent change in resistance is suitable fortriggering an alarm, although higher or lower changes are also suitablethresholds.

.If a security window is made with a continuous conductive layer overmost of the area of the window without any electrical isolation linessubdividing it into a plurality of conductive areas,'the change inresistance as a function of the'magnitude of the interruption of theconductive layer may be unduly low. When the entire window beneath thetwo bus bars constitutes a continuous conductive layer, each point oneach bus bar is in direct electrical contact with every point on theother bus bar. Current flow between the two bus-bars can therefore occurover a substantial area and destruction of a minor portion'of theconductive layer may have a relatively minor affect on the totalresistance. Thus, for example, in one test wherein the distance betweenthe 6 bus bars was 1.67 ti'mes the width of the conductiv layer in adirection parallel to the bus bars, a straight line cut was made throughthe conductive layer in a direction parallel to the bus bars. A cutextending more than 20 percent of the way between the side edges of theconductive layer increased the resistance less than l.1 percent. Acircular interruption in the conductive layer having a diameter of aboutl7 percent of the width of the conductive layer caused an increase inresistance of only about 2.6 percent.

When isolation lines are 'scribedth'rough the conductive layer in adirection extending between the bus bars the conductive filmis'divided'into a plurality of conductive areas that are electrically inparallel with each other. Then whena sufficient cut is made parallel tothe bus bars to completely sever one of such parallel conductive areas ajump'in resistance occurs. Thus, for example, a conductive layer wassubdivided intosix conductive areas by five scribed isolation lines. Asa straight line cut proceeded across one ofthe parallel conductive areasa nominal gradual change in resistance occurred. When one of the sixparallel conductive areas was completely severed-between adjacentisolation lines, an increase in resistance of about 20 percent wasobserved. Since a similar length cut in a film without isolation lineswould produce a resistance change of less than about 1% the value of theparallel conductive areas can be readily seen. In addition to increasingthe sensitivity of the security window to relatively small penetration,the sensitivity of the circuit for detecting achange in resistance canbe readily correlated with the resistance change that may occur when oneconductive area of a selected width is severed.

Since the isolation lines are aubstantially invisible the securitywindow may have its conductive layer subdivided-into any desired widthof conductive area for predetermined sensitivity. In a typical'window 30inches wide the conductive layer is divided into four segments, each 7%inches wide. Addition of only two more isolation lines cuts the width ofeach area to only 5 inches for very high sensitivity to penetration. Ifdesired a large number of electrical isolation lines can'be extendedbetween the bus bars so that the conductive layer is divided into anumber of narrow parallel conductors. A penetration of the windowinterrupts a number of such narrow conductors and the resistance changeis the usual change due to deleting some of the resistors in a parallelarray of resistors.

The array of resistors in electrical parallel is considered to extendover most of the area of the window since penetration at any point willinterrupt one or more resistors. This may be true even when theresistors become narrower than the electrical isolation lines betweenthem. Thus, if desired, one could form narrow strips of conductivematerial on the carrier film with clear areas between the strips andhave a structure differing only in scale from the arrangementillustrated in FIG. 1, for example.

FIG. 3 is a schematicillustration of a system for detecting penetrationof the security window. Very broadly the penetration of the conductivelayer causes an analog change in the resistance of the window which is afunction of the extent of penetration, and the magnitude of this changemay be used for triggering an alarm. The resistance of the conductivelayer 26 is represented by the resistor 26' is the schematicillustration of FIG. 3. This resistance is connected to the de- 7tecting circuit by electrical leads 33 including the window bus barsandwhatever additional leads may be desired for conveying signals to aremote location. The thin film resistor 26 is connected in a bridgewith-a resistor 34 as an adjacent arm of the bridge; A power supply 36applies-an electrical signal to the resistances 26' and 34.Theelectrical signal is also applied to a fixed resistor37 and a variableresistor'38 connected in series with atapped resistor or potentiometer39. These additional resistors 37, 38 and 39 form the other two arms ofa bridge. The variable resistor 38 maybe employed for a coarseadjustment of the bridge balance. Resistor 37 can be adjustable forbridge balance, too, or both resistors 37 and 38 may be coupled forcoarse bridge balance.

An amplifier 41 is connected between the adjacent bridge arms26' and 34and is also connected to the tap on the potentiometer 39. Adjustment ofthe tap can serve as a fine adjustment of the bridge balance. The bridgeexcitation provided by the power supply 36 can be either a voltage orcurrent arrangement. Similarly the power supply can be either AC or DCas may Many variations in the bridge excitation and unbalance detectionwill be apparent to one skilled in the art.

The output of the amplifier 41 is applied to a conventional thresholddetector 42. which senses when thenull balance of the bridge is-outsideof a predetermined limit. A widevariety of threshold detectors may besuitable, depending on the signal selected from the amplitier in aparticular embodiment. When the threshold detector notes that the bridgeis out of balance beyond the preset limit an alarm 43 is triggered. Anydesired alarm may be used such as a bell, klaxon, light or the like. Thealann can be adjacent the window 'or remotely located. One can evendispense with the threshold detector and apply the amplifier outputdirectly to an audio alarm, such as, for example, a loudspeaker. Whenthe sound of the loudspeaker reaches some arbitrary level as noted byan: individual in the vicinity this can also serve as an alarm.

Once an alarm has sounded, and it has been determined that the signal iserroneous or one decides to presently ignore the unbalance, thebridgecan bereadjusted by means of the resistors 38 and 39 to bring it backinto balance. This is quite feasible since the signal output from thebridge is analog. A change in the resistance of the conductive layermodifies the electrical signal in an analog manner. The alarm system cantherefore be reset by rebalancing the bridge, all of which canbe done"from a remote location if desired. Such isinfeasible in a window fittedwith conventional lead tapes or witha conductive layer directly on theglass since rupture of the tape or layer on glass is a binary output andthe circuit cannot be restored without access to the window and repairof the tape.

Some changes in the resistance of the conductive layer may occurgradually even when penetration of the window is not attempted, thus forexample, temperature changes in the window may cause resistance changesin the conductive layer of a sufficient magnitude to unbalance thebridge. One can therefore provide an automatic balance reset 44 whichsenses an unbalance and brings thebridge back to null by adjusting thepotentiometer 39. The fact of resetting of the bridge or the magnitudeof the resetting may be recorded with a conventional recorder 45. Thecumulative change in resistance recorded by the recorder 45 could beused to trigger an alarm if desired. It will be apparent, of course,that the balance reset 44 may be operated by the amplified nullbalancesignal from the amplifier 41 so as to operate in a more analogfashion and accommodate slow drifts in theresistance balance of thebridge. Similarly, if desired the balance reset or bridge adjustment maysimply control operation of the amplifier 41 to remain below thethresholdsignal. Electrical balance of the amplifier can substitute; foractual bridge resistance balancing-for alarm actuation as well. Theautomaticreset 44 and recorder 45 are not essential to the functioningofthe system.

It will also be apparent that high degree of sophistication mayalsobeincorporated in the balance detection system sothat, for example,a single transient of resistance can be ignored and a more permanentchange employed for triggering the alarm. Means may alsobe providedfortriggering the alarm in case the bridge leads are shortedor cut, or ifthe power is cut off, or if any of a variety of techniques are employedfor, circumventing the alarm system. i

' As mentioned above, the resistance of the conductive layer in thewindow may vary with temperature and cause an unbalance of the bridge;Although this can be readily accounted for with an automatic balanceresetting system, it is also" quite easy'tosimply compensate for thetemperature change by making the fixed resistor 34 in the adjacent armof the bridge to the resistor 26' also be a conductive layer in awindow. If the temperature pattern in the two resistors is similar, anychanges in resistance will be equivalent and the balance of the bridgewill not be upset. The second conductive layer in a window may be in aseparate window located in a position subject to similar temperatureconditions or it may simply be another portion of the same window inwhich the layerresistor 26' islocated.

FIG. 13 illustrates in face view another embodiment of security windowhaving a conductivelayer extending over-"most of the area of the window.In this embodiment, thereis a bus bar 102 extending along one side edgeof the window formaking electrical contact with one entire edge of theconductive layer in the window. An electrical isolation line 103 extendsacross the window transverse to the bus bar 102 and divides theconductive area of the window into two conductive regions 104 and 106. Abus bar 107 extends part way along the side edge of the window oppositefrom the full length bus bar 102 and makeselectrical contact with theconductive layer of the first region 104. A second similar bus bar 108extends the balance of the way across the window and makes electricalcontact with" the conducbars 107 and 108. Resistors 111 and 112 are alsoconnected to the power supply. A first tap 113 is connected between theresistors 111 and 112 and a second tap 114 is connected to the bus bar102 that makes electrical contact with both conductive regions 104 and106. The taps 113 and 114 may be connected to any conventional nullbalance direction circuitry as desired for triggering an alarm inresponse to unbalance of resistance. It will be noted that the windowand external circuit illustrated in FIG. 13 are connected as aconventional bridge with the two conductive regions of the window asadjacent arms of the bridge. Either or both of the resistors 111 or 112can be variable for balancing the bridge, or balancing can be achievedin the additional circuits (not shown) to which the window may beconnected.

- The two conductive regions of the window will both be subjected tosimilar temperature conditions and any changes in resistance in the tworegions will be similar. Being in adjacent bridge arms, the resistancedrift due to temperature change balances out and no bridge unbalanceresults. It will be apparent that if desired the I conductive layer ineach of the conductive regions 104 first bus bar 51 is in electricalcontact along'an edge of the first conductive area 46. A similar shortbus bar 52 is in electrical contact with an edge of the other outsideconductive area 49. A third bus bar 53 is'in electrical contact with theedges of both of the remaining two conductive areas '47 and 48 spanningone of the isolation lines 50. Along the opposite edge of the securitywindow from the first three bus bars is a fourth bus bar 54 inelectrical contact with the edges of the two conductive areas 46 and 47.Another bus bar 55 on this same edge-of the window is in electricalcontact with the edges of the remaining two conductive areas 48 and 49.Suitable conductive tabs 56 extend from the bus bars to and beyond theedges of the window for making electrical contact to external circuits.If desired, the conductive areas between the isolation lines 50 can alsobe subdivided into parallel resistive areas in the same manner as thewindow of FIG. 1 for enhanced sensitivity.

FIG. illustrates schematically the interconnection of the conductiveareas. The bus bars and conductive areas of FIG. 4 are representedschematically with the same reference numerals bearing a prime in FIG.5. The two bus bars 51 and 52 are externally interconnected at a point51', 52. This same point is connected to a suitable power supply 57which is in turn connected to the center bus bar 53 at a point 53' inthe schematic illustration of FIG. 5. Electrical connection is made tothe opposite bus bars 54 and 55 at the points 54 and 55 leading to adetector 58 of changes in the electrical resistance. The detector 58 canbe connected for triggering an alarm 59 when a predetermined liminalchange in electrical resistance occurs in the bridge formed by theresistors (conductive areas) 46', 47', 48', and 49'.

Since the four resistors or conductive areas are connected as the fourarms of a resistance bridge, any resistance changes occurring in all ofthe conductive areas ductive layer within the security window changesresistance somewhat with changing temperature. Similarly, stresses onthe conductive layer which may be generated by bending on the window,for example, may changeresistance. When the conductive areas areinterconnected as arms of a bridge such thermal or stress changes inresistance do not cause false alarms. It will be apparent to one skilledin the art that if desired more than one window may be interconnected asarms of a resistance bridge and the conductive areas may sufficientlybalance to compensate for thermal changes and the like. This isgenerally less desirable since the thermal changes or changes in stressbetween two windows is usually of much greater magnitude than similarchanges within two different areas of the same window. It will also-beapparent that the resistance change detector 58 should be adjustable forresetting the alarm system in case of a permanent change in theresistance balance between the arms of the bridge.

FIG. 6 illustrates in face view another embodiment of I security window.In, the embodiment of FIG. 1, the conductive areas were electricallyconnected in parallel. In the embodiment of FIG. 6, the conductive areasare connected in series. Thus, as illustrated in this embodiment, aplurality of isolation lines 61 extend between opposite edges of thesecurity window and subdivide the area into a plurality of conductiveareas 62. Electri- I cal contact is made along a side edge of one of theoutside conductive areas by a bus bar 63. A tab 64 permits electricalconnection of this bus bar to an external circuit. At the opposite endof the first conductive area from the bus bar 63 is a second bus bar 65which makes electrical contact along the side edge of the firstconductive area and also along the side edge of the second conductivearea adjacent the first. That is, the second bus bar 65 spans theisolation line 61 between the two adjacent conductive areas. Another busbar 66 electrically connects the opposite side edge of the secondconductive area with the side edge of the third conductive area. Theseadditional bus bars 65 and 66 do not have an external tab for electricalconnection to circuits outside the window.

A similar series of additional bus bars connect adjacent conductiveareas clear across the window. In the final conductive area a bus bar 68makes electrical connection to both the edge of the conductivearea and atab 69 permitting electrical connection to an external circuit. Thus,all of the conductive areas within the window are electrically connectedtogether in series. Clearly a penetration that extends through the fullextent of one of the conductive areas will interrupt the continuouscircuit and provide a substantially infinite increase in resistance.Such an electrical connection is not resettable from a remote location.

If desired a tab 70 may be provided on a central bus bar for makingcontact to an external circuit thereby permittingv half of theconductive areas to be in one arm of a bridge and the other half inanother arm of a bridge for temperature and stress compensation. Such aseries connected security window is very sensitive to smallpenetrations. The same effect can be obtained without the large numberof bus'bars by simply ending alternate isolation lines a substantialdistance from each of the opposite edges of the window respectively. Apattern of isolation lines for a series-parallel connection ofconductive areas can also be used.

As mentioned hereinabove the security window is highly sensitive topenetrations that have a component extending in a direction parallel tothe bus bars. If the interruption in the conductive layer is primarilyin a direction between the bus bars, that is, for example, parallel tothe isolation lines, little if any change in resistance is observed.Thus, for example, if the object of penetration of the security windowis the passage of contraband, a narrow slit extending between the busbars may be sufficient for the unlawful purpose without causing asufficient change in resistance to trigger the alarm. This possibilityis effectively forestalled with a security window of the typeillustrated in FIG. 7.

As illustrated in this embodiment at least the central portion of thesecurity window has two spaced apart conductive layers extending overmost-of the area of the window. A first pair'of bus bars 72 are providedalong the opposite side edges of the security window in electricalcontact with the edges of one of the conductive layers. Orthogonal tothis first set of bus bars is a second pair of bus bars 73 in electricalcontact with the edges of the second conductive layer within the window.The isolation lines in the conductive layers between the opposed busbars have-been deleted from FIG. 7 for enhancing clarity of the drawing.It will be readily apparent that no penetration of the window can bemade that does not have a component parallel to one or the other of thetwo pairs of bus bars. It is therefore substantially impossible topenetrate such a window with any reasonable size hole without triggeringan alarm.

The arrangement of bus bars in the window illustrated in FIG. 7 is assimple as possible and, if desired, arrangements such as illustrated inFIGS. 4 and 6 may be employed. The two conductive layers can be employedas a pair of arms in a bridge, or portions of thetwo conductive layersmay be used as the four arms of a bridge. There is a'possibility,although remote, that penetration of both layers could causecompensating resistance changes in the two layers when they are used asadjacent arms of a bridge. The two layers can be used as opposite armsof the bridge so that penetration of both layers causes an increase insensitivity.

FIG. .8 illustrates in fragmentary cross section a laminate securitywindow having two conductive layers therein. In this illustrationsuccessive layers are cut away to best show the location of the busbars. in this particular example the arrangement of successive layers issymmetrical from the center of the laminate, however, it will beapparent that asymmetrical arrangements are also suitable.

Each face of the laminated security window comprises a glass ply 75. Aplastic interlayer 76 about 0.03 inch thick is bonded to each of theglass plys 75. A carrier film 77 of polyethylene terephthalate about0.005 inch thick and having a thin conductive metal coating 78 thereonis bonded to each of the interlayers 76. Centrally located in thelaminate is. a third interlayer 79 which bonds the two carrier filmstogether. One of the bus bars 72 is imbedded in one of the plasticinterlayers 76 so as'to be in electrical contact with one of theconductive layers 78. The bus bar 72 is illustrated schematically ratherthan show. the corrugations of the preferred bus bar hereinabovementioned. The other bus bar 73 is imbedded in the opposite interlayer76 so as to be in electrical contact with the other conductive layer 78.During fabrication of such a laminated window initially flat sheets ofpolyvinyl butyral for the interlayers are assembled in a sandwich andduringthe heat and pressure cycle of lamination this relatively softmaterial deforms so that the respective bus bar imbeds therein. Theeffect of plural layers for detecting penetration can also be obtainedbe depositing thinmetal films on both faces of the carrier film andlaminating that carrier in a window with bus bars in contact with bothmetal layers.

FIG. 9 illustrates in fragmentary cross section another embodiment ofsecurity window suitable for use in locations where customary access isalmost entirely on one side of the window; This laminated structure hasa glass ply 81 on one face, such as, for example, one quarter inchtempered glass. A polyvinyl butyral interlayer 82 is bonded to theglass. A'carrier film 83 having a thin conductive metal layer (notshown) thereon is bonded to the plastic interlayer 82. A secondinterlayer- 84 bonds the carrier film 83 to a relatively thick'ply oftransparent polycarbonate plastic 86. The 'exposedface of the plasticply 86 is coated witha protective layer 87 such as chemically depositedsilica, titania or the like, which affords a substantial degree ofabrasion resistance and protection against chemical attack on plastic.

Such an asymmetrical laminated security window may be used, for example,in an institution wherein the glass layer 81 is used onthe inside wherethe inhabitants have access to the window. The plastic layer would.

FIG. 10 illustrates a security window such as might be used fortemporary purposes. In this embodiment a pair of carrier films 89 havingthin conductive metal layers 90 thereon are bonded together with aplastic layer 91 which may be a polyvinyl butyral interlayer ashereinabove described or may be other suitable adhesive bonding. Therelatively thick interlayer is not generally needed in such a situationsince its ability to conform to the rigid glass and polycarbonate plysof the other embodiments is not a requiremenL'Care must be taken, ofcourse, to insulate the two metal layers 90 from each other if both areused in active alarm circuits. Similarly the bus bars (not shown) makingcontact to the con ductive layers 90 must be insulated. It will also beapparent that if desired a carrier film having a metal coating thereoncan be adhesively bonded to a similar film which serves to protect thedelicate metal layer from damage and a security window suitable fortemporary use may be very inexpensively provided. Bus bars are needed tomake contact with the metal layer. Such a security window made with thinplastic films has considerable flexibility and is light in weight makingit 'quite suitable for temporary use. Such a lamination of plastic filmswith a conductive layer therein can be bonded on a window and connectedto suitable detection and alarm circuits for forming a security window.Preferably the conductive layer in such a security window is scribedwith electrical isolation lines and electrically connected in a bridgecircuit in one of the manners hereinabove described.

There is a distinct advantage in the arrangement illustrated in FIG. 2wherein the conductive layer 26 is separated from the tempered glass ply23 by at least the interlayer 24 and, if desired, the carrier film 25.The plas- 'tic isolates the conductive film from the glass layer so thatif the glass is merely broken most, if not all, of the conductive layerremains intact. It is a characteristic of tempered glass that a breakpropagates over the entire extent of the glass breaking it into a verylarge number of small fragments. If the conductive layer wereon theglass or closely coupled thereto such breakage of the glass wouldcompletely rupture the delicate metal layer and it would appear thatpenetration was being attempted. With the metal layer decoupled from theglass by a relatively soft resilient intervening plastic layer merebreakage of the glass does not disrupt the conductive film to more thana minor extent. Even if an alarm might be sounded when the temperedglass is broken the alarm system can be reset to indicate when anattempt is made to penetrate the window.

The resistance of thethin metal film embedded in the laminated securitywindow is also sensitive to strain. That is, as the film is strained,the resistance changes. Thus, for example, when the conductive layer islocated off, of a neutral axis of the cross section of the laminatedwindow, a bending of the window will induce strain in the conductivelayer and change its resistance. This discovery gives one anopportunityto employ the deformation of the window prior to penetration forproviding an alarm signal. More particularly a transient change inresistance can be detected with a pulse or rate of change greater thansome'predetermined-magnitude. r

FIG. 11 illustrates in block diagram a system for utilizing the strainsensitive properties of the conductive film'in a laminated window forproviding a security alarm. A power supply 93 applies power to theconductive metal layer in a security window 94. The excitation appliedto the window by the power supply may be AC or DC,'and may be eithercurrent or voltage as desired. The window 94 is also connected to'an ACamplifier 96 with gain control-97. The AC amplifier may also have a bandwidth control, if'desired, for limiting the AC range amplified. Theoutput of the AC amplifier is applied to a threshold detector 98 whichapplies an outof-limits signal to an alarm 99'. y

if someone should commence striking or otherwise deforming the window inorder to effect penetration, the resultant time varying signal isamplified. ,When the v signal is within the frequency band of the ACamplified and beyond the preset threshold, an alarm will sound. Such asystem is also responsive to the rate of change of penetration through awindow as represented by the changing resistance. In an impact sensitivesystem one can use the magnitude of the pulse of changing resistance todetect a penetration or attempt at penetration. If desired the rate ofchange or rise time of the pulse or pulse width can be selected fortriggering an alarm. The circuitry for detecting any such characteristicof changing resistance is conventional.

The sensitivity of the threshold detector 98 can be set so that thestrain required to activate the alarm 99 is a large fraction of thestrain that would occur in the win dow before breakage. Gain control 97may effect this function. Thus, relatively minor blows on a window whichare far short of causing breakage can be ignored and a pulserepresenting a sufficient blow to be quite near breakage of the windowcan be detected. One can monitor strain amplitude and detect pressuresthat are a large fraction of the force required to break the window.With AC amplification in the system a blow that is sufficient to raisethe strain level to breakage will activate the alarm and indicatepenetration. It has been noted in penetration tests of a window having athin metal conductive film that during the penetration event very highexcursions in resistance occur and thereafter the resistance settles toan equilibrium value characteristic of the response to severance of thefilm. Thus, for example, penetration of the security window by a highspeed projectile will cause a large change in resistance as penetrationoccurs which can be detected by the AC amplification system. Thus thepenetration can be detected by the rapid pulse of resistance change,even though the steady state resistance may not be significantlydifferent from the original, resistance. 7

It may be desirable to employ a strain sensitive detection system as setforth in FIG. 11 with a penetration sensing system as illustrated inFIG. 3. In such an arrangement the input to the AC amplifier 96 may beeither the output of the null balance bridge of FIG. 3 or the output ofa DC amplifier 41 which may reduce the gain requirements of the ACamplifier 96.

If desired, a continuous surveillance monitor 10] may be connected tothe output of the AC amplifier 96. This continuous surveillance monitormay have a visual or aural output so that an attendant can perceivesignal changes, such as, for example, due to someone pounding on asecurity window. The continuous surveillance monitor can also be somemeans for recording the output signal for review at a later time.

A resistance bridge is of course not the only way of detecting a changein the resistance of the conductive layer. A simple and inexpensivetechnique is illustrated in FIG. 12. As illustrated in this arrangementa voltage e, is applied to a resistor 102 connected to an input of anoperational amplifier 103. A second resistor 104 is connected across theamplifieL'The output voltage e,, is proportional to the input voltageand the ratio of the resistance of resistor l04 to the resistance ofresistor 102. The output voltage is thus quite sensitive to any changein the relative values of the two resistors. A conductive layer in asecurity window can be used for either of the two resistors in thisschematic diagram, or

if desired two conductive areas in a window could be used as bothresistors 102 and 104 for temperature compensation. The operationalamplifier can be either a single input amplifier, or can be adifferential amplifier with two inputs.

One can also use ohmmeter circuits or a variety of current or voltagecomparison circuits for noting a change in resistance due to windowpenetration. It will also be noted that the resistance values can bedigitized at any point in the electrical circuit and digital techniquesused for balancing, comparing and the like. If the resistance of aconductive layer is digitized it can be compared to a digital referencenumber and the balance can be maintained by changing the referencenumber. As much or as little sophistication as desired can be achievedwith digital techniques for comparison, correction and avoidance offalse alarms or tampering with the system. Many other arrangements willbe apparent to one skilled in the art for detecting a variation inresistance of the security window.

means connected to the means for monitoring for giving an output signalin response to a rapid change in resistance such as to produce, at theoutput of said means for monitoring, a pulse having a predeterminedpulse characteristic in excess of a predetermined threshold.

2. A security window system as defined in claim 1 wherein the means forgiving an output signal comprises .an AC amplifier connected to theelectrically conductive layer.

3. A security window system as defined in claim 2 v wherein saidmeansfor giving an output signal comprises-a threshold detector connected tothe AC ampli-,

fier and an alarm responsive to the threshold detector.

4. A security window system as'defined in claim 2 wherein the means forgiving an output signal includes means for producing an analog outputsignal perceptible by a person.

5. A security window system as defined in claim 2 further comprisingmeans for adjusting the means for giving an output signal for providingan output signal only when the magnitude of the pulse exceeds apredetermined fraction of the pulse that would occur upon penetration ofthe window. T 6. A security window system as defined inclaim 5 furthercomprising means connected to the conductive layer for giving an outputsignal in response to an accumulated change in resistance in excess of apredetermined magnitude.

7,. A security window system comprising: an electrically conductivetransparent layer extending over most of the area of the window, saidlayer having an electrical property changeable in response to strain ofthe window; I means connected to the conductive layer for detecting achange in the electrical property; and means connected to the means fordetecting for giving an output signalwhen the strain of the windowexceeds a predetermined limit. 8. A security window system as defined inclaim 7 wherein the means for detecting comprises means for detectingaztransient'change in resistance of the conductive layer in excess of apredetermined limit.

9. A security window system as defined in claim 7 wherein the conductivelayer comprises a thin metal film deposited on a transparent carrierfilm and wherein the carrier film is laminated inthe window off film inexcess of a predetermined limit.

11. A security window system as defined in claim 7 further comprisingmeans connected to the electrically. conductive layer for giving anoutput signal when the equilibrium resistance of the layer exceeds apredetermined limit.

I l i l i l

1. An impact sensitive security window system comprising: a transparentelectrically conductive layer extending over most of the area of thewindow; means for monitoring resistance of the conductive layer; andmeans connected to the means for monitoring for giving an output signalin response to a rapid change in resistance such as to produce, at theoutput of said means for monitoring, a pulse having a predeterminedpulse characteristic in excess of a predetermined threshold.
 2. Asecurity window system as defined in claim 1 wherein the means forgiving an output signal comprises an AC amplifier connected to theelectrically conductive layer.
 3. A security window system as defined inclaim 2 wherein said means for giving an output signal comprises athreshold detector connected to the AC amplifier and an alarm responsiveto the threshold detector.
 4. A security window system as defined inclaim 2 wherein the means for giving an output signal includes means forproducing an analog output signal perceptible by a person.
 5. A securitywindow system as defined in claim 2 further comprising means foradjusting the means for giving an output signal for providing an outputsignal only when the magnitude of the pulse exceeds a predeterminedfraction of the pulse that would occur upon penetration of the window.6. A security window system as defined in claim 5 further comprisingmeans connected to the conductive layer for giving an output signal inresponse to an accumulated change in resistance in excess of apredetermined magnitude.
 7. A security window system comprising: anelectrically conductive transparent layer extending over most of thearea of the window, said layer having an electrical property changeablein response to strain of the window; means connected to the conductivelayer for detecting a change in the electrical property; and meansconnected to the means for detecting for giving an output signal whenthe strain of the window exceeds a predetermined limit.
 8. A securitywindow system as defined in claim 7 wherein the means for detectingcomprises means for detecting a transient change in resistance of theconductive layer in excess of a predetermined limit.
 9. A securitywindow system as defined in claim 7 wherein the conductive layercomprises a thin metal film deposited on a transparent carrier film andwherein the carrier film is laminated in the window off of the neutralaxis of the window cross section.
 10. A security window system asdefined in claim 9 wherein the means for detecting comprises means fordetecting a transient change in resistance of the metal film in excessof a predetermined limit.
 11. A security window system as defined inclaim 7 further comprising means connected to the electricallyconductive layer for giving an output signal when the equilibriumresistance of the layer exceeds a predetermined limit.